The energy and cost efficiency of aluminum smelting can be significantly reduced with the use of inert, non-consumable and dimensionally stable anodes. Replacement of traditional carbon anodes with inert anodes should allow a highly productive cell design to be utilized, thereby reducing capital costs. Significant environmental benefits are also possible because inert anodes produce no CO.sub.2 or CF.sub.4 emissions. The use of a dimensionally stable inert anode together with a wettable cathode also allows efficient cell designs and a shorter anode-cathode distance, with consequent energy savings.
The most significant challenge to the commercialization of inert anode technology is the anode material. Researchers have been searching for suitable inert anode materials since the early years of the Hall-Heroult process. The anode material must satisfy a number of very difficult conditions. For example, the material must not react with or dissolve to any significant extent in the cryolite electrolyte. It must not react with oxygen or corrode in an oxygen-containing atmosphere. It should be thermally stable at temperatures of about 1,000.degree. C. It must be relatively inexpensive and should have good mechanical strength. It must have high electrical conductivity at the smelting cell operating temperature, e.g., about 950.degree.-970.degree. C., so that the voltage drop at the anode is low. In addition, aluminum produced with the inert anodes should not be contaminated with constituents of the anode material to any appreciable extent.
Some examples of inert anode compositions are provided in U.S. Pat. Nos. 4,374,050, 4,374,761, 4,399,008, 4,455,211, 4,582,585, 4,584,172, 4,620,905, 5,794,112 and 5,865,980, assigned to Aluminum Company of America. These patents are incorporated herein by reference.